Novak Djokovic is 27, six years the junior of Roger Federer. Serena Williams will soon be 33, same as Federer.

Time marches on, sparing no one. So while experience remains a significant factor for an elite stage such as the U.S. Open, it's worth looking at younger players who could find themselves thrust into championship contention.

But what of Bulgaria's Grigor Dimitrov and Milos Raonic of Canada? Both are 23 and growing in prowess. Dimitrov and Raonic, according to John McEnroe, as "the two most obvious guys ready to step in the void."

Dimitrov is so often compared to Federer in his prime he is being called "Baby Federer." Recent strides in strength and conditioning make his stock soar, and observers are eager for a potential quarterfinal match between the master and pupil. Now living in Los Angeles, he is the son of a tennis pro who is building on his 6-3 frame.

A Financial Times interview earlier this month gave a closer look at Dimitrov, who left home at 13 for advanced training. About being called "Baby Federer": Dimitrov understandably isn't overjoyed. He said he believes in results and prefers to left them speak for him.

The real question for Dimitrov and Raonic: Are they ready? Sports Illustrated looked at the men's field, and its writers have their doubts.

More experienced, Australian Open winner Stan Wawrinka is 29. So is Jo-Wilfried Tsonga, who appears to be peaking at the right time. Andy Murray, a former Open champ, is 27 and could be a challenger were it not for questions about his back and just where coach Amalie Mauresmo has him entering the tournament.

Stephens has been viewed as the successor to Serena, and she thrives in the spotlight. While American men's tennis is in perhaps its deepest doldrums ever, Stephens is among women who are rising. At 19, Madison Keys is a power player with big upside.